The present invention relates to immobilized free radical captors, to their preparation and to their use. More specifically, the present invention relates to:
immobilized free radical captors, and especially immobilized radical polymerization inhibitors or retarders;
a method of preparing them; and
a method of stabilizing, at least temporarily, molecules or mixtures of molecules, which makes use of said immobilized captors.
According to prior art, free radical captors are used per se in various contexts for protecting molecules which are sensitive to free radicals, from said free radicals. They are suitable, for example:
as stabilizers or preservatives: placed in the presence of sensitive molecules, such as biological molecules (for example, polyunsaturated lipids, vitamins), they minimize the degradation of the sensitive molecules by said radicals;
as radical polymerization inhibitors or retarders: dispersed within monomers, they prevent any uncontrolled and/or premature polymerization thereof (especially during phases of storage and transport of said monomers). Such radical polymerization inhibitors or retarders (methoxyphenol, phenothiazine, para-benzoquinone, 2,2-diphenyl-1-picrylhydrazyl (DPPH), metal salts) and their mechanisms of action (in the presence of monomers such as acrylic acid, butyl acrylate, acrylonitrile, styrene, even more generally, acrylates and methacrylates) have notably been described:
by L. B. Levy in Journal of Polymer Science; Polymer Chemistry, 23, 1505 (1985) and 30, 569 (1992) (methoxyphenol, phenothiazine, acrylic acid monomer);
by L. B. Levy in Journal of Applied Polymer Science, 60, 2481 (1996) (methoxyphenol, butyl acrylate);
by S. S. Cutixc3xa9, D. E. Henton, C. Powell, R. E. Reim, P. B. Smith, T. L. Staples, in Journal of Applied Polymer Science, 64, 577 (1997) (methoxyphenol, acrylic acid); and
in Encyclopaedia of Polymer Science and Engineering, 2nd edition, vol. 13, p. 729-735, Ed Wiley Interscience (1988) (para-benzoquinone, DPPH, metal salts, acrylates, methacrylates, acrylonitrile, styrene . . . ).
Thus, the commercially available liquid monomers, especially those of the vinylic type (acrylates, or styrene, for example) usually contain 50 to 1,500 ppm of dissolved inhibitor(s) or retarder(s). Said inhibitor(s) or retarder(s) rapidly react with any free radical which is spontaneously generated at ambient temperature and/or under the action of light.
According to a first variant, such molecules prevent any efficient polymerization, and it is imperative to get rid of them when it is desired to initiate said polymerization. Their prior and compulsory removal, by distillation or chromatography, is a relatively difficult technique to carry out, especially on an industrial scale. The complete removal is in any case only with difficulty.
According to a second variant, molecules of this type are only active as inhibitors or retarders up to a certain temperature threshold and/or up to a certain irradiation threshold. It is necessary to, pass over this threshold in order to initiate the polymerization desired in the presence of said molecules. In such a context, said polymerization, deferred with time, is slower to carry out, more difficult to control and, in any case, it generates a polymer which contains said molecules as impurities.
The presence of these molecules in the monomers is furthermore cumbersome or restricting, for various reasons. Thus
in order to maintain a certain constancy in the process of polymerization, especially in its kinetics, it proves to be compulsory to keep the concentration of said molecules constant. The latter, obviously, decreases with time: the longer the storage time is, the more significant the amount of inhibitor(s) or retarder(s) consumed is. Maintaining said amount constant is a real constraint;
certain inhibitors or retarders, such as hydroquinone or methoxyphenol, only react with the free radicals in the presence of oxygen. It is however difficult to ensure a homogeneous dissolved oxygen content within a mixture of monomers. Failing this, the polymer obtained by polymerization of said mixture will not have a perfect homogeneity. The Applicant is especially confronted with this technical problem during the preparation of photochromic or non-photochromic lenses; the leaktightness at the joint of the lens mould is never perfect;
said inhibitor molecules may induce parasitic reactions with various types of other molecules (photochromic or non-photochromic colorants, chromophores . . . ), which may be present in the mixture to be polymerised;
said inhibitor molecules can also directly influence the final properties of the polymer prepared. Thus, they may alter the optical properties thereof. For a typical concentration of 100 ppm of said inhibitor molecules in the mixture to be polymerised, said molecules, which contain Cxe2x80x94H and Cxe2x80x94OH groups, increase the loss of transmission at 1.55 xcexcm, by about 0.1 dB/cm in the polymerised material . . .
Upon considering all the problems set forth above, the Applicant desired developing an alternative to the use in accordance with prior art of dissolved free radical captors. The Applicant proposes to no longer use said dissolved free radicals captors per se, but to make them immobilized on a solid support. Thus, whatever their nature and the context of their use may be, they can be easily manipulated, in general got rid of, prior to the implementation of the polymerization, in the particular case of radical polymerization inhibitors or retarders.
Thus, according to a first object, the presently claimed invention relates to free radical captors, especially radical polymerization inhibitors or retarders within the sense of the prior art which, characteristically, are immobilized on a solid support.
Said free radical captors, especially of the stabiliser type (preservative type) or polymerization inhibitor or retarder are, according to the invention, fixed in a stable manner onto a solid support.
Advantageously, the radical polymerization inhibitors or retarders are selected from:
phenol derivatives, and especially:
alkylphenols, such as 2,6-di-tert-butyl-4-methylphenol,
hydroquinone,
alkoxyphenols, such as methoxyphenol,
catechol and derivatives thereof, such as 4-tert-butylcatechol;
quinones and especially benzoquinone;
phenothiazine;
organic radicals known as stable organic radicals, and especially:
nitroxy radicals, such as 2,2,6,6-tetramethylpiperidinooxy (TEMPO),
2,2-diphenyl-1-picrylhydrazyl (DPPH);
nitro derivatives, and especially:
nitromethane,
nitrobenzene;
metal salts, and especially:
CuBr2,
FeCl3, both used in solution, advantageously in dimethylformamide (DMF);
sulphur derivatives, used in iniferter systems.
These radical polymerization inhibitors or retarders are known per se. Within the context of the present invention, it is proposed to use them in an original manner, i.e. immobilized on a support.
The most-used radical polymerization inhibitors or retarders to this day are phenol derivatives and stable organic radicals. According to a preferred variant, the free radical captors, immobilized in the sense of the invention, consist of said phenol derivatives or said stable organic radicals.
The solid support which intervenes can be of any nature. Obviously, it must be suitable for the stable immobilization of the captor on its surface, as well as for the further use of said immobilized captor. Generally, it is a mineral and/or organic support, more frequently a mineral or organic support, and, preferably, a mineral support.
An organic support, either by nature or after chemical modification of its surface, possesses on said surface numerous reactive functions, notably of the alcohol-, amine-, carboxylic acid-, halide-, ester-, amide-type . . . These reactive functions are advantageously used for the immobilization (coupling), generally by chemical grafting, of the captors.
The intervention of a:
silica support or silica-based glass support (containing more than 50% by weight of silica);
an alumina support, or
an iron oxide support, titanium oxide support . . . is recommended as mineral support.
This list is not exhaustive.
The immobilization of the captor on such mineral supports is generally carried out via a covalent bond, via silanes, boranes, zirconates, alumino-zirconates, titanates, or equivalents. A chemical grafting is more particularly recommended, via a silane, on a silica support or a silica-based support.
The intervening solid support can furthermore have various forms. These can notably be particles, recipient walls, or detachable structures . . .
Thus, according to the invention, the free radical captor is advantageously immobilized on particles which are intended to intervene dispersed in liquids. Said particles are advantageously as fine as possible and also advantageously have a surface/volume ratio (a specific surface) as great as possible. However, their size must be sufficient, on the one hand, in order to enable the immobilization of the captor at their surface easily, and on the other hand, in order also to easily enable their physical separation (for example, by filtration or centrifugation) from the liquid medium in which, dispersed, they exert their action (notably as radical polymerization inhibitor). Thus, said particles generally have a size between 20 nm and 50 xcexcm, advantageously between 200 nm and 1 xcexcm.
The person skilled in the art has already grasped the interest of this variant of the invention. Such particles, on the surface of which free radical captors are immobilized, are perfectly suitable for the stabilization of monomers (stored, transported) awaiting polymerization. When it is desired to carry out said polymerization, it is easy to remove said particles, much easier to remove said particles than the free radical captors which intervene per se, according to prior art.
The free radical captors can also intervene, as indicated above, on the walls of a recipient. Inside said recipients, the products which are sensitive to free radicals can thus be protected. Such recipients can notably consist of reactors (within which, for example, it is desired to functionalise the molecules, notably monomers, protected from free radicals), storage reservoirs, transport reservoirs, piping or analogues. It can prove to be particularly interesting to have effective captors immobilized onto the internal walls of piping.
The concept of the inventionxe2x80x94immobilization of free radicalsxe2x80x94can also be developed according to another variant: said free radical captors intervening immobilized on detachable structures. Said free radical captors are therefore of great flexibility of use. The detachable structure can, as much as is desired, be introduced and then taken out of the medium in which the immobilized free radical captors must exert their action. The detachable structure can notably be successively introduced and then taken out of recipients of the type set forth above. Said detachable structure advantageously has a significant specific surface (a surface/volume ratio). Advantageously, it is a cellular and/or porous structure.
The concept of the invention can therefore avail itself according to numerous variants, notably with reference to the nature and the form of the intervening support.
It is now proposed to specify, in a totally non-limiting way, how the free radical captors can, according to the invention, be immobilized onto the solid support. In the absolute, any type of stable coupling is suitable. Obviously, it is suitable to conserve, during use, the captor immobilized on the support in order not to come up against the problems encountered according to prior art with dissolved captors.
The free radical captor immobilized according to the invention onto the solid support can especially be linked to said support:
via a covalent bond, or
via weaker bonds, such as ionic bonds, hydrogen bonds . . .
The first of these variants is particularly preferred, i.e. a chemical grafting (via a covalent bond). Said chemical grafting may or may not make use of a coupling agent.
In this context of chemical grafting, i.e. by making use of a covalent bond, the captor/support bond can even be developed according to numerous variants, such as:
a direct grafting of said captor onto the non-treated, non-modified surface of said support;
a direct grafting of said captor onto the surface of said support having undergone a surface treatment (for example: plasma treatment or chemical treatment of the oxidation type or others);
an indirect grafting of said captor onto the surface of said support, the surface being optionally treated; said indirect grafting making use of a coupling agent; it being possible for various types of coupling agent to be used, notably xe2x80x9cshortxe2x80x9d coupling agents and xe2x80x9clongxe2x80x9d coupling agents, which are intrinsically long or which make use of a spacer . . .
The person skilled in the art upon considering the nature of the support and the nature of the captor knows how to carry out efficient chemical graftings.
As has been indicated already:
the reactive functions of the alcohol-, amine-, carboxylic acid-, halide-, ester-, amide-type . . . , of organic supports are advantageously made use of for such chemical graftings;
coupling agents of the silane-, borane-, zirconate-, alumino-zirconate-, and titanate-type are themselves advantageously used for captor/mineral support chemical graftings.
Whatever the type of support, the use of a spacer is generally prioritised, insofar as the captor, grafted via a xe2x80x9clongxe2x80x9d coupling agent, gains mobility and is therefore thus capable of best expressing its properties. The optimal expression of said properties can obligatorily be required in certain contexts. It is generally as such of the radical polymerization inhibitors or retarders used for stabilizing difunctional monomers; difunctional monomers such as divinylbenzene, which are more sensitive to free radicals than monofunctional monomers such as (meth)acrylates.
Generally, when a coupling agent is made use of, the immobilization of the captor on the surface of the support is carried out in two steps:
either the captor and the coupling agent are firstly joined, and then secondly, said coupling agent, joined to said captor is grafted onto the surface of the support:
or the coupling agent is firstly grafted onto the surface of the support, and then secondly, the captor is joined to said coupling agent grafted onto said support.
In the context of an advantageous variant of the invention, the captor is immobilized by chemical grafting (via a covalent bond), onto a silica support or a silica-based glass support (vide supra); a coupling agent, advantageously of the silane type, joining said captor to said support.
The captor immobilized in the sense of the invention can be schematized thus:
SupportSiO2xe2x88x92Coupling agent(Silane)xe2x88x92Captor.
Said coupling agent of the silane type or non-silane type, joined or not joined to the captor, has in general been grafted onto the surface of the support by condensation (grafting technique, per se, familiar to the person skilled in the art).
This same type of chemical reactionxe2x80x94condensationxe2x80x94may be carried out with other coupling agents (vide supra) onto surfaces of the silica type or silica-based type, even onto other types of surface (based on other metal oxides).
Purely as an illustration, coupling agents which are advantageously used according to the invention for immobilizing free radical captors onto a silica or silica-based mineral support can be listed below:
silicon tetrachloride (SiCl4), alone: short coupling agent;
tetramethoxysilane (Si(OCH3)4), alone: short coupling agent;
silicon tetrachloride (SiCl4)+bromopropanol (CH2Brxe2x80x94CH2xe2x80x94CH2OH): long coupling agent (short coupling agent+spacer);
glycidoxypropyl trimethoxysilane 
intrinsically long coupling agent.
It is recalled here that the intervention of xe2x80x9clongxe2x80x9d coupling agents is generally prioritised. This remark applies more particularly to silane-type coupling agents.
Thus, within the context of the advantageous variant of the invention specified above, the free radical captor is preferably capable of linking to the silica of the support by a group comprising a chain having at least three atoms different from silicon, a chain of the type:
xe2x80x94Sixe2x80x94Oxe2x80x94Cxe2x80x94C . . . , or
xe2x80x94Sixe2x80x94Cxe2x80x94Cxe2x80x94C . . .
The use of a spacer between the Si of the coupling agent and the free radical captor is more particularly recommended. Thus, advantageously, said captor can be grafted onto the silica via a group of the second of the types above, i.e. of the xe2x80x94Sixe2x80x94Cxe2x80x94Cxe2x80x94Cxe2x80x94 type . . . ; insofar as the Sixe2x80x94Cxe2x80x94 bonds are more resistant to hydrolysis than the Sixe2x80x94O bonds. This remark is of course valid whatever the length of the group may be.
It is recalled here that the person skilled in the art will perfectly master the chemistry which enables carrying out the graftings of the various types set forth above.
According to a second object, the present invention relates to a method of preparing said immobilized free radical captors. Said method comprises immobilizing, advantageously via a chemical grafting, either directly or via a coupling agent, said captor onto the surface of a solid support having adequate reactive groups.
It has been seen above that said immobilization can be carried out according to numerous variants, said variants obviously being adapted to the nature of said solid support. It has especially been seen that when said immobilization results from a chemical grafting, it is advantageously carried out by making use of a coupling agent, especially of a silane; said coupling agent being condensed onto the surface of the support. Details on this aspect of the method of the invention are given above, during the description of the product arising from said method: the grafted captor.
Finally, according to a third object, the present invention relates to the use of said grafted captor, namely a method of stabilizing, at least temporarily, molecules or mixtures of molecules which are sensitive to free radicals. Said method comprises placing, at least temporarily, said molecules or mixtures of molecules in contact with an effective amount of at least one grafted free radical captor. It is of course understood that in general, several free radical captors intervene which may be of the same nature or not . . .
The following is specified with reference to the expression xe2x80x9cat least temporarilyxe2x80x9d as used above. It is of course obvious that the invention was developed for a use to a temporary stabilization end. The introduction to the present text may be referred to. It is however also obvious that the use of the captors immobilized in the sense of the invention is not excluded for xe2x80x9cdefinitivexe2x80x9d protection ends (throughout the whole of the life of the product protected).
Said process, the third object of the presently claimed invention, may avail itself according to numerous variants, and in numerous contexts. Notably, the process may comprise:
incorporating and dispersing particles, on which at least one free radical captor is immobilized, within molecules or mixtures of molecules to be stabilized;
maintaining said particles within said molecules or mixtures of molecules for their stabilization (at this stage in the method, it can be a case of an intrinsically stable suspension or of a stabilized suspension, for example by agitation or by electrostatic interaction);
physically separating said particles from said molecules or mixtures of molecules for recovering said particles on the one hand, and on the other hand, said molecules or mixtures of molecules, thus rendered sensitive once again to free radicals.
Advantageously, the free radical captors are, in this context, radical polymerization inhibitors or retarders. They intervene temporarily for example within a mixture of monomers during its storage and/or transport. They are then easily removed for example by centrifugation or filtration, prior to carrying out the polymerization of said mixture of monomers.
In other contexts, such radical polymerization inhibitors or retarders are immobilized on recipient walls or on detachable structures. In order to annihilate their inhibitory action with regard to monomers, they are removed from contact with them. In the first case, the monomers are transported away, in the second case said detachable structure is removed.
The person skilled in the art, upon reading the foregoing, has not failed to grasp the interest of the invention. The invention is now illustrated by Examples A and B below.
Within the context of said Examples, various methods of immobilizing radical polymerization inhibitors or retarders onto mineral supports (silica particles) have notably been set forth. For reasons of simplicity, only the term xe2x80x9cradical polymerization inhibitorxe2x80x9d is spoken of in said Examples. The person skilled in the art cannot ignore that according to the kinetics in question, this term reveals to be suitable or not. In this latter case, it is hereafter advantageously implicitly replaced by the term xe2x80x9cradical polymerization retarderxe2x80x9d.